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RWLOCK(9)	       FreeBSD Kernel Developer's Manual	     RWLOCK(9)

     rwlock, rw_init, rw_init_flags, rw_destroy, rw_rlock, rw_wlock,
     rw_runlock, rw_wunlock, rw_try_upgrade, rw_downgrade, rw_sleep,
     rw_initialized, rw_wowned,	rw_assert, RW_SYSINIT -- kernel	reader/writer

     #include <sys/param.h>
     #include <sys/lock.h>
     #include <sys/rwlock.h>

     rw_init(struct rwlock *rw,	const char *name);

     rw_init_flags(struct rwlock *rw, const char *name,	int opts);

     rw_destroy(struct rwlock *rw);

     rw_rlock(struct rwlock *rw);

     rw_wlock(struct rwlock *rw);

     rw_runlock(struct rwlock *rw);

     rw_wunlock(struct rwlock *rw);

     rw_try_upgrade(struct rwlock *rw);

     rw_downgrade(struct rwlock	*rw);

     rw_sleep(void *chan, struct rwlock	*rw, int priority, const char *wmesg,
	 int timo);

     rw_initialized(struct rwlock *rw);

     rw_wowned(struct rwlock *rw);

     options INVARIANTS

     rw_assert(struct rwlock *rw, int what);

     #include <sys/kernel.h>

     RW_SYSINIT(name, struct rwlock *rw, const char *desc);

     Reader/writer locks allow shared access to	protected data by multiple
     threads, or exclusive access by a single thread.  The threads with	shared
     access are	known as readers since they only read the protected data.  A
     thread with exclusive access is known as a	writer since it	can modify
     protected data.

     Although reader/writer locks look very similar to sx(9) locks, their
     usage pattern is different.  Reader/writer	locks can be treated as
     mutexes (see mutex(9)) with shared/exclusive semantics.  Unlike sx(9), an
     rwlock can	be locked while	holding	a non-spin mutex, and an rwlock	cannot
     be	held while sleeping.  The rwlock locks have priority propagation like
     mutexes, but priority can be propagated only to an	exclusive holder.
     This limitation comes from	the fact that shared owners are	anonymous.
     Another important property	is that	shared holders of rwlock can recurse,
     and exclusive locks can be	made recursive selectively.

   Macros and Functions
     rw_init(struct rwlock *rw,	const char *name)
	     Initialize	structure located at rw	as reader/writer lock,
	     described by name name.  The description is used solely for
	     debugging purposes.  This function	must be	called before any
	     other operations on the lock.

     rw_init_flags(struct rwlock *rw, const char *name,	int opts)
	     Initialize	the rw lock just like the rw_init() function, but
	     specifying	a set of optional flags	to alter the behaviour of rw,
	     through the opts argument.	 It contains one or more of the	fol-
	     lowing flags:

	     RW_DUPOK	   Witness should not log messages about duplicate
			   locks being acquired.

	     RW_NOPROFILE  Do not profile this lock.

	     RW_NOWITNESS  Instruct witness(4) to ignore this lock.

	     RW_QUIET	   Do not log any operations for this lock via ktr(4).

	     RW_RECURSE	   Allow threads to recursively	acquire	exclusive
			   locks for rw.

	     rw_rlock(struct rwlock *rw)
			   Lock	rw as a	reader.	 If any	thread holds this lock
			   exclusively,	the current thread blocks, and its
			   priority is propagated to the exclusive holder.
			   The rw_rlock() function can be called when the
			   thread has already acquired reader access on	rw.
			   This	is called ``recursing on a lock''.

	     rw_wlock(struct rwlock *rw)
			   Lock	rw as a	writer.	 If there are any shared own-
			   ers of the lock, the	current	thread blocks.	The
			   rw_wlock() function can be called recursively only
			   if rw has been initialized with the RW_RECURSE
			   option enabled.

	     rw_runlock(struct rwlock *rw)
			   This	function releases a shared lock	previously
			   acquired by rw_rlock().

	     rw_wunlock(struct rwlock *rw)
			   This	function releases an exclusive lock previously
			   acquired by rw_wlock().

	     rw_try_upgrade(struct rwlock *rw)
			   Attempt to upgrade a	single shared lock to an
			   exclusive lock.  The	current	thread must hold a
			   shared lock of rw.  This will only succeed if the
			   current thread holds	the only shared	lock on	rw,
			   and it only holds a single shared lock.  If the
			   attempt succeeds rw_try_upgrade() will return a
			   non-zero value, and the current thread will hold an
			   exclusive lock.  If the attempt fails
			   rw_try_upgrade() will return	zero, and the current
			   thread will still hold a shared lock.

	     rw_downgrade(struct rwlock	*rw)
			   Convert an exclusive	lock into a single shared
			   lock.  The current thread must hold an exclusive
			   lock	of rw.

	     rw_sleep(void *chan, struct rwlock	*rw, int priority, const char
			   *wmesg, int timo)
			   Atomically release rw while waiting for an event.
			   For more details on the parameters to this func-
			   tion, see sleep(9).

	     rw_initialized(struct rwlock *rw)
			   This	function returns non-zero if rw	has been ini-
			   tialized, and zero otherwise.

	     rw_destroy(struct rwlock *rw)
			   This	functions destroys a lock previously initial-
			   ized	with rw_init().	 The rw	lock must be unlocked.

	     rw_wowned(struct rwlock *rw)
			   This	function returns a non-zero value if the cur-
			   rent	thread owns an exclusive lock on rw.

	     rw_assert(struct rwlock *rw, int what)
			   This	function allows	assertions specified in	what
			   to be made about rw.	 If the	assertions are not
			   true	and the	kernel is compiled with	options
			   INVARIANTS and options INVARIANT_SUPPORT, the ker-
			   nel will panic.  Currently the following assertions
			   are supported:

			   RA_LOCKED	Assert that current thread holds
					either a shared	or exclusive lock of

			   RA_RLOCKED	Assert that current thread holds a
					shared lock of rw.

			   RA_WLOCKED	Assert that current thread holds an
					exclusive lock of rw.

			   RA_UNLOCKED	Assert that current thread holds nei-
					ther a shared nor exclusive lock of

     locking(9), mutex(9), panic(9), sema(9), sx(9)

     These functions appeared in FreeBSD 7.0.

     The rwlock	facility was written by	John Baldwin.  This manual page	was
     written by	Gleb Smirnoff.

     If	WITNESS	is not included	in the kernel, then it is impossible to	assert
     that the current thread does or does not hold a read lock.	 In the
     non-WITNESS case, the RA_LOCKED and RA_RLOCKED assertions merely check
     that some thread holds a read lock.

     Reader/writer is a	bit of an awkward name.	 An rwlock can also be called
     a ``Robert	Watson'' lock if desired.

FreeBSD	9.2		       November	25, 2007		   FreeBSD 9.2


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